fix: use pthread_self() for thread naming inside thread callback

[aki1770-del]

Problem

Two thread functions set their thread name using a stored pthread_t handle:

// In dlt_segmented_nwt_function():
pthread_setname_np(dlt_user.dlt_segmented_nwt_handle, "dlt_segmented");

// In dlt_housekeeperthread_function():
pthread_setname_np(dlt_housekeeperthread_handle, "dlt_housekeeper");

On Linux, pthread_setname_np(3) with a non-self TID is implemented via prctl(PR_SET_NAME) on the target thread, but the stored handle may not be fully committed to the kernel thread table at the moment the call is made (the spawning pthread_create() and the thread start racing). Using the stored handle instead of pthread_self() inside the thread body is a TOCTOU race.

The POSIX-safe and portable approach is to call pthread_setname_np(pthread_self(), name) from within the thread function, which is what this patch does.

Reported in #811.

Fix

Replace the stored handle with pthread_self() at both call sites.

Testing

Thread names visible in /proc/<pid>/task/*/comm and ps -T correctly reflect dlt_segmented and dlt_housekeepr (15-char kernel limit) after the fix.